Sound translating systems



Dec. l0, 1957 R. M.A cARRELL SOUND TRANSLATING SYSTEMS Fgled Nov. so,1954 Fz'y. I,

5 Sheets-Sheet l INVENTOR.

BY Z Z Dec. 10, 1957 R. M. cARRr-:LL 2,816,165

SOUND TRANSLATNG SYSTEMS Filed Nov. 30, 1954 3 Sheets-Sheet 2 maarAfrox/VH Dec. 10, 1957 R. M. CARRELL 2,816,165

souND TRANSLATING SYSTEMS 3 Sheets-Sheet 3 Filed Nov. 30, 1954 TOHNEXSOUND TRAN SLATIN G SYSTEMS Ross M. Carrell, Audubon, N. J., assigner toRadio Corporation of America, a corporation of illelaware AppiicationNovember 30, 1954, Serial No. 471,951

8 Claims. (Cl. 179-1) This invention relates to sound translatingsystems, and more particularly to sound translating systems for higherorder gradient operation.

Radio broadcasting in general and television broadcasting in particular,as well as motion picture recording, generally require directionalmicrophones which discriminate against unwanted sound, such asreverberant sound or unavoidable background noises. The directivity rofa microphone is one of the main characteristics which determines thedistance between the microphone and a performer in a given environment.In many cases, the directivity of some microphones is such that themicrophone must be placed within a few feet of the performer, just outof camera range. In television broadcasting, these limitations ofmicrophones constitute a problem of some proportions.

A large increase in ease and ilexibility of programing may be eiected bya substantial increase in directivity, providing that this is notaccompanied by a large increase in bulk. The bulk of some types ofdirectional microphones has limited their use to a few long rangeoutdoor pickup applications. Other types of directional microphonesrequire complex arrays of matched microphone elements. Such arrays,usually have low overall sensitivity.

In numerous other applications, outside of the television field, it isdesirable to cut or to reduce background noises. These noises may befrom operating machinery in factories or from airplane engines, forexample. High order gradient systems have a marked axial noisediscrimination as well as increased directivity, providing an immunityto noise fields which is superior to that obtainable from pressure orfirst order gradient microphones.

Much of the theory relating to higher order gradient sound translatingsystems has been well established. A pressure gradient responsivemicrophone is one in which the output is substantially proportional to aderivative of sound pressure with respect to distance from the source.Microphones of this type are classified according to the order of thepressure derivative. Thus, for example, a first order microphone has anoutput proportional to the first derivative. A second order microphonehas an output proportional to the second derivative. An nth ordermicrophone has an output proportional to the nth derivative. A firstorder pressure gradient responsive, or velocity type, microphone maycomprise either two elements responsive to the pressure of a sound wave,or a single element responsive to the pressure gradient of the soundwave. A second order pressure gradient responsive microphone may includeeither two rst order microphones or four pressure responsivemicrophones. A third order microphone may include either two secondorder microphones or four first order microphones, or eight pressureresponsive microphones. An nth order microphone may include 2n pressureresponsive microphones. A fuller discussion of unidirectional and higherorder-gradient microphones may be foundin Elements `of AcousticalEngineering by Harry F. Olson, second edinited States Patent tion, 1947,on pages 253 to 276. It is understood that single elements in a systemmay, in effect, be the equivalent of a plurality of elements.

Among the major considerations which have prevented wlde 4commercialacceptance of higher order gradient microphones is a requirement thatthe frequency response and the sensitivity of the microphone elements bevery closely matched. Another consideration has been that the responseof microphones or microphone arrays utilizlng higher order gradientoperation is ordinarily not independent of frequency. A thirdconsideration which has prevented wide use of higher order gradientmicrophones has been the relatively low overall sensitivity of the highorder microphone array.

It is believed that many of the disadvantages inherent in higher ordergradient microphones arise from an assumption that the amplifying systemmay only amplify the signal from the microphone, and conversely, thatany special properties, such as increased directivity, must belong tothe microphone alone. If the microphone and the amplifier system areconsidered as a cooperative unit, then effective solutions to theproblems presented by higher order gradient microphone systems may beattained.

It is an object of this invention to provide a novel sound translatingsystem for higher order gradient operation.

It is a further object of this invention to provide a sound translatingsystem for higher order gradient operation in which the number ofmicrophones required is minimized.

In accordance with the present invention, a sound translating system forhigher order gradient operation is provided. An electric eld whosedirection, magnitude and space distribution of phase angle is analogousto the corresponding quantities in a sound field is created in anelectrically conductive element. This eld is attained by placing aplurality of microphones in the sound eld and applying the electricaloutputs from the microphones to the electrically conductive element.Once the electric iield is created, a plurality of electrodes may beapplied to various points on the electrically conductive element. Theelectrodes may then be connected to a utilization circuit. Theelectrical outputs from the electrodes will then be closely equivalentto electrical outputs which would be obtained from microphones placed atpoints in the sound eld which correspond to the position of theelectrodes on the electrically conductive element. Thus, in effect, theelectrical contacts ltake the place of microphones.

Other objects and advantages of the present invention will be apparentand suggest themselves to those skilled in the art to which theinvention relates, from a reading of the following specification inconnection with the accompanying drawing, in which:

Figure 1 is a curve representing a sound wave in a sound field;

Figure 2 represents a sound translating system in accordance with thepresent invention;

Figure 3 is a curve representing an electric eld in an electricallyconductive element corresponding to a portion of the sound waveillustrated in Figure l; and

Figures 4 to 9 show embodiments of a sound translating system inaccordance with the present invention.

Referring particularly to Figures l, 2 and 3, a curve 10 represents theinstantaneous pressure distribution of a sound wave traveling in thedirection of the arrow, as indicated. A plurality of microphones 12, Mand 1.6 are represented as being disposed along a straight line in thedirection ofthe sound wave. The microphones 12 and 16 may be spacedapproximately one-sixth or less of the wave length 7\ of the transmittedwave. The sound pressures at the microphone positions are translatedinto corresponding electrical signals by the microphones. The electricalsignals from the microphones 12 and 16 are assumed to be applied in somemanner to an element 18 which has the physical property of presentingvoltages throughout its length which correspond to instantaneouspressures at corresponding points in the sound field. The element 18,for purposes of this discussion of principles of the present invention,may be considered as a conductor. This conductor may be in the nature ofa wa re transmission system composed of lumped elements.

If the microphones are of the pressure sensitive type the electricalsignals applied to the ends of the electrically conductive element 18will correspond to the instantaneous pressures in the sound wave fieldat the positions in said field occupied by the microphones 12 and 16.The microphone 14, placed at a position in the sound field will producea voltage which is intermediate between that produced by the microphones12 and 16. In a plane wave sound field this voltage will be equal inmagnitude to that produced by microphones 12 and 16, but the phase angleof the voltage will be between that of the voltages produced by themicrophones 12 and 16.

lf the electrically conductive element 18 is arranged so that there mayexist at different places in or on the element, voltages which areintermediate in phase as well as voltage between the voltages applied tothe ends of the element 18, then it can be said that there exists withinthe element an analogue of the external sound field existing between themicrophones 12 and 16.

Consider that the conductive element 18 is one which will permit theestablishment of an electric eld which is analogous to the externalsound field existing on a line drawn between microphones 12 and 16. Itwill be seen that within the body of the electrically conductive element18 there will exist a point at which the voltage and phase willcorrespond to that which would be obtained from the microphone 14. Thisis illustrated diagrammatically in Figure 2, where 01 is equal to 01.

Thus, it will be seen that an electrode placed in contact with theelectrically conductive element corresponds and is analogous to amicrophone placed in the external sound field.

Referring particularly to Figure 4, a pair of pressure operatedmicrophones 22 and 24 are disposed in a sound field. Spacing between themicrophones may be approximately M6 or less. An electric fieldcorresponding to the sound pressure wave in the sound field is createdin an electrically conductive element 26. A pair of electrodes 2S and 30touches or is otherwise electrically connected to the electricallyconductive element 26. The electrical outputs from the contacts may besubtracted in any well known manner. In the embodiment shown,subtraction is attained by connecting the electrical outputs from thecontacts in phase opposition. It is seen that the output from the twocontacts 28 and 30 will be substantially equivalent to a first orderpressure gradient microphone having pressure responsive elements locatedin the sound field corresponding to the electric field within theelectrically conductive element.

A pair of contacts 32 and 34 are also electrically connected to theelectrically conductive element 26. The electrical outputs from thecontacts are subtracted in any Well known manner, such as by connectingthe outputs in phase opposition, as indicated. The electrical outputsfrom the contacts are substantially equivalent to electrical outputs ofa pair of pressure sensitive microphones disposed at points in the soundfield corresponding to the electrode positions in the electric field.The electrical outputs from each of the two pairs of contacts may beconsidered substantially equivalent to the output of a first order orvelocity type microphone.

In order to attain second order operation within the sound systemillustrated, the subtracted output from the pair of contacts 28 and 30are subtracted from the subtracted output of the pair of contacts 32yand 34. Again, the subtraction is attained by connecting the outputs inphase opposition. Any order of higher order gradient operation deisredmay be attained by increasing the number of contacts and employingfurther subtracting operations.

It is thus seen that once an electric field is created which correspondsto a sound field, electrical contacts may be substituted formicrophones. Thus the number of microphones required for higher ordergradient operation is greatly reduced. The necessity of matchingcharacteristics of more than two microphones is thereby eliminated.

If a unidirectional or other type of response characteristic is desired,a phase shifting network may be incorporated into the sound translatingsystem. One such arrangement is illustrated in Figure 5. A pair ofmicrophones 36 and 38 are spaced approximately M 6 or less apart in asound field. The electrical output of the microphone 38 is applied to yaphase shifting network, illustrated by a block 40. Such phase shiftingnetworks are known in the acoustic field and, consequently, the phaseshifting network 40 is not described in detail. The electrical outputfrom the phase shifting network is applied to one end of an electricallyconductive element 42. The electrical output from the microphone 36 isapplied to the other end of the conductive element 42. The voltages fromthe microphone 36 and the phase shifting network 40 creates an electricfield within the element 42. The electrical eld created correspondssubstantially to the existing sound field between the microphones 36 and38. The phase shifting network may be designed to attain a substantiallyuni-directional characteristic in the sound system.

A pair of contacts 44 and 46 are electrically connected to theconductive element 42. The electrical outputs from the contacts areconnected in phase opposition, as indicated. A pair of contacts 48 and50 are also electrically connected to the conductive element 42 withtheir electrical outputs being connected in phase opposition. Theelectrical output from each of the contacts substantially corresponds toan electrical output of a pressure sensitive microphone disposed at 'apoint in the sound field corresponding to the electric field within theconductive element 42.

The electrical outputs from the contacts 48 and 50 are first subtractedfrom each other and then applied to a phase shifting network 52. Theelectrical output from the phase shifting network 52 is then subtractedfrom the combined electrical output of the contacts 44 and 46. Again,the subtraction is achieved by connecting the electrical outputs inphase opposition'.

The combined outputs from the phase shifting network 52 and the contacts44 and 46 provide a second order gradient operation characteristic. Theexact shape and directional response of the system shown will depend toa large extent upon the types of phase shifting networks employed.Systems for orders higher than two is attainable through the use of alarger number of contacts and additional subtracting operations.

Referring particularly to Figure 6, there is illustrated means forcreating an electrical analogue corresponding to the sound pressure in asound field. A pair of microphones 54 and 56 which may, for example, beof the pressure sensitive type, are disposed in a sound field. Thespacing may be less than a wave length apart, preferably 'y/ 6 or lessat the frequency to be reproduced.

In order that the system may attain an actual perform ance which isclose to that theoretically obtainable, it is necessary that the outputsof the microphones 54 and 56 be equal in magnitude for equal soundpressures at every frequency within the range being considered. This maybe also stated as requiring that the frequency response characteristicsand the sensitivitiesv of the two microphones be as similar as possible.

In practical manufacture it is simpler to match the frequency responsesof two microphones than it is to match both the. frequency response andthe sensitivity. Considering that in a practical case that themicrophones 54 and 56 have identical frequency response characteristicsbut different sensitivities, this may be compensated for by a properadjustment of the gains of the amplifiers 58 and 6i), which may be donemanually or automatically.

The amplified signals from the amplifier 60 arel applied to a phaseshifting network 64. The phase shifting network may be employed toprovide the desired directional characteristic within the soundtranslating system. The signal voltages from the amplifier 5S and thephase shifting network 64 are applied across opposite ends of aresistance network 66.

The resistance network 66 may be considered as a phase divider. Theoutput voltage from the amplifier 58 is applied between an input tap 68and a point of reference potential 70, hereinafter referred to asground. The signal output from the phase shifting network 64 is appliedbetween a tap 72 and ground.

Consider that the amplifier 58 and the phase shifting network 64 haveequal and low output impedances (compared to the tapped resistance 76).Consider also that voltages of equal magnitudes but unequal phase appearat the terminals of the amplifier 58 and phase shifting network 64. ltwill be shown that voltages taken between intermediate points on theresistor 76 and ground will have phase angles between the phase anglesof the amplifier 5S and the phase shifting network 64.

Consider, first the voltage appearing between the output terminal 63 ofthe amplifier 58 and ground. This is essentially the output of theamplifier 58 alone, for it has been stated that the resistance 74 ismuch greater than the output impedance of the amplifier 58.

Similarly, it will be seen that voltage between the terminal 72 andground will be essentially that due to the phase shifting network 64alone.

At an intermediate point on the resistor 76, the voltage to ground willbe the vector sum of the voltages contributed to that point by theamplifier 53 and the phase shifting network 64. The relativevcontribution of each is determined by the point where the voltage toground is taken.

`lf negligible current is drawn from a tap on` the resistor, and if thevoltages appearing at terminals 68 and 72 are equal in magnitude, and ifthese voltages are less than )t/ 6 apart, it will be found that thephase angle of the voltage between the tap and ground varies nearlylinearly with the ratio of the resistance between thetap and one end tothe total resistance. Also the magnitude of the voltage will be constantwithin i2 db as the tap is moved from one end to the other. Thevariation in phase and magnitude of the voltage is symmetrical about themidpoint of the resistor 76.

It is apparent that this phase .division network satisfies theconditions of an analogue of the sound field between the microphonesSland 56. The usefulness of this analogue is limited to total phaseshifts of roughly 100 degrees or less; but this is not a limitation inthe present system which is limited by other considerations to phaseshifts less than 90.

It is seen that the phase relationship of the electrical voltage atdifferent points on the resistor 76 may correspond to the phase of acorresponding acoustical wave in a sound field within which themicrophones 54 and 56 are disposed. Consequently, electrical outputsfrom electrical contacts placed at various points along the resistor 76will be substantially the same iu form as electrical outputs taken frommicrophonesdisposed in a corresponding sound eld between the microphones54 and 56. The principles described herein and employed in the presentinvention may be employed to attain higher order gradient operation in asound translating system without 6 the necessity of a large number ofmicrophones with close ly matched characteristics.

The voltage across a tap 7f3 and ground is applied to an amplifier 77,illustrated here as being a triode electron discharge device comprisinga grid 79, a cathode and an anode 82. Suitable operating potential forthe amplifier is supplied by a source indicated by B+. The amplifiedelectrical signal from the amplifier 77 is applied across one half of acenter tapped output transformer 84.

The voltage output across a tap 36 and ground is applied to an amplifier88, which comprises a triode electron discharge device having a grid 9),a cathode 92 and an anode 94. The amplified elctrical signals from theamplifier 33 is then applied to one half of the output transformer 34.It is seen that the electrical outputs from the amplifiers 77 and 88 areconnected in phase opposition across the transformer 84 to provide meansfor subtracting the outputs. This provides an arrangement equivalent toa first order gradient sound system.

It is seen that the voltages appearing at the taps 70 and 86 will be ofdifferent phase relationships depending upon the phase relationship ofthe sound field within which the microphones 58 and 60 are disposed.

The voltage appearing across a tap 96 and ground is applied to anamplifier 98 comprising a triode electron discharge device having a gridlili), a cathode 102 and an anode 1912-. The amplied signal voltage isapplied across one half of a center-tapped output transformer 106.

The voltage appearing across a tap 163 and ground is applied to theamplifier comprising a triode having a grid 112, a cathode 114i and ananode 116. The amplified signal voltage from the amplifier 11@ isapplied across one half of the center-tapped output transformer 106. Itis seen that the output signal voltages from the amplifiers 93 and 110are connected in phase opposition thereby providing a subtractingoperation. This arrangement provides means for attaining first ordergradient operation in the sound system. Suitable operating potentialsfor the amplifiers 93 and 11@ are provided by a source designated as B+.

In order to attain second order gradient operation, the signal voltagesdeveloped across the secondary windings 115 and 117 of the transformers84 and 106, respectively, are subtracted from each other. The signalVoltage from the secondary winding 117 of the transformer 106 is appliedto a phase shifting network 118. The signal output from the phaseshifting network 113 is applied to an amplifier 120. The signal outputfrom the secondary winding 115 of the transformer 84 is also applied tothe amplifier 120. The secondary windings 115 and 117 are connected sothat the developed signal voltages thereacross are in phase oppositionto provide a subtracting operation. The subtracting operation providesan `arrangement for second order gradient operatron.

After the output signal voltage from the secondary Winding 115 of thetransformer tid is subtracted from the signal voltage of the phaseshifting network 118, the subtracted or combined voltage is amplified bythe amplifier 120 and then applied to an equalizing network 122. Thisequalizing network compensates for the rising frequency responseinherent in a higher order gradient system. The reasons for this risingresponse are well known to the art and may be found in any standardreference work on the subject. The essential function of the equalizingnetwork 122 is to provide a substantially flat overall frequencyresponse characteristic from the entire system.

This arrangement illustrating one embodiment of the present inventionhas provided an electrical analogue corresponding to a sound field in aresistor '76, and may 1n some respects be considered as one dimensional.

Referring to Figure 7, there is illustrated another means for creatingan electrical analogue corresponding to the sound pressure within asound field. A plurality of microphones 124, 126, 128 and 130 aredisposed in a sound field. The acoustic signals picked up by themicrophones are translated into corresponding electrical signals. Theelectrical signals from the microphones are applied across a pluralityof plates 132, 134, 136, and 138, respectively, and ground. The plateselectrically Contact a sheet of electrically conductive material 14) andare suitably spaced at convenient points. The electrical signal outputsfrom the microphones create an electric field within the material 140.The electric field created may be made to correspond to theinstantaneous pressure of the sound waves within the sound field.

A plurality of electrodes 142, 144, 146 and 14S are suitably mounted toa block 149, which may be an electrically insulated material. Theelectrodes may be disposed at various points on the surface of thematerial 140. The placement of the electrodes will depend upon theparticular directional characteristic desired. Leads 141, 143, 145 and147 are connected to the electrodes 142, 1M, 146 and 14S, respectively.The leads maybe connected to utilization circuits, not shown.

To attain higher order operation, the electrical outputs from theelectrodes 146 and 14S may be subtracted from each other. Likewise, theelectrical outputs from the electrodes 142 and 144 may also besubtracted from each other. The subtracted outputs from the two sets ofelectrodes 142, 144 and 146, 148 may be subtracted from each other toattain second order operation. This subtraction operation may becontinued to attain third, or even higher order gradient operation. Thenumber of electrodes must be increased as the order of operationincreases. Means for amplifying and subtracting the voltages may besimilar to those illustrated in Figure 6, and consequently are not againdescribed in detail.

Itis noted that the contacts on the conductive material 149 provide thesame functions as microphones disposed in a sound field. Consequently itis seen that the `directional or other output characteristics of thesound illustrated may be varied by varying the positions of the contactson the material 14). Thus it is not necessary to move a plurality ofmicrophones to attain certain desired outputs when, for example, aperson in front of a television or movie camera moves to differentpositions.

Referring particularly to Figure S, there is illustrated another meansfor creating an electrical analogue for a sound eld. A plurality ofmicrophones 15d, 152, 154 and 156 are disposed in a sound field. Theacoustical signals picked up by the microphones are translated intocorresponding electrical signals. The voltages from the microphones 150,152, 154 and 156 are applied to a plurality of plates or contacts 157,158, 159 and 169, respectively. The plates are electrically connected toa mass of electrically conductive material 162. The signal voltages fromthe plates produce an electric field within the mass of conductivematerial 162. A plurality of electrodes 164, 166, 168 and 171i aredisposed to electrically contact the mass 162. The electrodes may besuitably mounted on a block 172, which maintains a constant desiredspacing between the contacts. The block 17.2 may be suitably attached toa shaft 174. The shaft may be designed to rotate within a mounting 176about the axis of the mass of the conductive material 162. The mounting176 is attached to the material 162 in any suitable manner.

The electrical signal voltages from 'the electrodes 164, 16o', 16d and171i are suitably applied to leads 163, 15, 167 `and 169. The leads maybe connected to utiliza tion circuits (not shown) and may be utilized in`any desired manner, such as previously indicated. in effect, theelectrodes applied to the electric field takes the place of microphonesdisposed in corresponding positions in the sound field. Thus variabledirectional characteristics are attainable from the sound system byrot-ating the positions of the electrodes rather than moving a pluralityo microphones.

Referring particularly to Figure 9, there is illustrated means forcreating a three-dimensional electrical analogue corresponding to asound field. A plurality of microphones 178, 180, 182, 184, 18.6 and 188are disposed within a sound field. The. electrical outputs from themicrophones are connected to plates 190, 192, 194, 196, 198 and 200,respectively. The plates electrically contact a container 202. Thecontainer includes an electrolytic liquid 204 therein. An electric eldcorresponding substantially to the sound field within which themicrophones are disposed is created within the electrolytic liquid 204.A plurality of electrodes 206, 298, 210 and 212 are mechanically mountedto a block 213 and disposed within the liquid 264. The block 213 isattached to one end of a shaft 14, the other end of which is rotatablymounted to the top of the container 202. The block 213 is pivotallymounted to the shaft 214 on the pivot 215. A bent rod 2.17 is attachedto the free end of the block and extends through the inside of theshaft. The shaft has a slot opening (not shown) to permit movement ofthe rod up or down. Movement of the rod changes the angular position ofthe block 213, with respect to the shaft 214. The angular positions ofthe electrodes 206, 2&18, 210 and 212 will consequently also be changedwhen the rod is moved. Leads 216, 218, 220 and 222 are connected to theelectrodes 206, 20S, 210 and 212, respectively. The leads may beconnected to utilization circuits, not shown. The voltages applied tothe contacts from the electrolytic liquid 204 may then be used in anydesired manner, such as, for example, in the manner previously indicatedto attain higher order gradient operation within a sound system.

Numerous other means and arrangements may be employed for creating anelectric field corresponding to a sound field. Once the field iscreated, it is seen that it may be utilized in many ways for creatingsound systems involving higher order gradient operation. Such soundtranslating systems eliminate the necessity of using a large number ofmicrophones which must be critically matched and substitutes thereforsimple electrical contacts. The critical matching of microphonecharacteristics is thereby eliminated. ,Since the number and matchingrequirements of microphones are minimized, it is seen that the cost ofhigher order gradient operation sound systems is greatly reduced.

For some purposes it may be desired to create special directivitypatterns for the systems by warping the electric field in the analogue.This can be done by using electrically conductive :material of specialshapes, or of non-uniform resistivity per unit area.

Special effects may also be created by using a field material whichpossesses an interval time delay. Such a material might be a highdielectric constant material, such as barium titallate, coated by a lmof resistive material and backed by a conductive base.

What is claimed is:

l. A sound ,translating system comprising means for convertingacoustical signals into corresponding electrical signals, an electricwave transmission element to provide yan electric field thereinanalogous to the sound field producing said electrical signals, meansfor applying said electrical signals to said element, means forelectrically contacting a plurality of lpoints on said element, autilization circuit, and means for electrically connecting saidplurality of points to said utilization circuit.

2. A sound reproduction system comprising a plurality of soundtranslating devices disposed in a sound field for converting acousticalsignals into corresponding electrical signals, an electric wavetransmission element, means for applying said electrical signals to saidelement to create an electrical field therein, said electrical eldsubstantially 4corresponding in phase -to said acoustical signals insaid sound field, mean-s for electrically contacting va plurality .ofpoints on said element, a utilization 9 circuit, and means forelectrically connecting said plurality of points to said utilizationcircuit.

3. A sound translating system comprising two microphones for convertingacoustical signals into corresponding electrical signals, an impedanceelement having resistive phase shift characteristics and thereby beingof a character to transmit said electrical signals without shifting thephase thereof, means for applying said electrical signals from each ofsaid two microphones to opposite ends of said element, means forelectrically contacting a plurality of points on said impedance element,a ultilization circuit, and means for electrically connecting saidplurality of points to said utilization circuit.

4. A sound reproduction lsystem comprising a plurality of soundtranslating devices disposed in a sound field for converting acousticalsignals into electrical signals, an impedance element, means forapplying said electrical signals to said element -to create anelectrical field therein, said electrical eld substantiallycorresponding in phase to said acoustical signals in said sound eld, apair of electrodes for electrically contacting a pair of points on saidelement whereby an electrical voltage appears in each of saidelectrodes, means for subtracting the voltage of one of said electrodesfrom the other of said electrodes, a utilization circuit, and means forelectrically connecting the substracted voltage from said electrodes tosaid utilization circuit.

5. A sound reproduction system for higher order gradient operationcomprising a plurality of sound translating devices for convertingacoustical signals into corresponding electrical signals, an impedanceelement, means for applying said electrical signals from said soundtranslating devices to said element, a plurality of pairs of electrodesfor electrically contacting a plurality of points on said elementwhereby electrical voltages having different phase relationships appearin each of said electrodes, means for producing a voltage representingsuccessive subtraction of the voltages of each of said pairs ofelectrodes, a utilization circuit, and means for lapplying thesubtracted voltage from said electrodes to said utilization circuit.

6. A sound reproduction system comprising a plurality of soundtranslating devices disposed in a sound eld for converting acousticalsignals into corresponding electrical signals, a sheet of electricallyconductive material, means for applying said electrical signals to saidsheet to create an electrical eld therein, said electrical eldsubstantially corresponding in phase to said acoustical signals in saidsound field, electrodes for electrically contacting a plurality ofpoints on said sheet, said signal applying means being l adapted to bemoved to dilerent points on the surface of said sheet, a utilizationcircuit, and means for electrically connecting said plurality of pointsto said utilization circuit whereby the voltages at said electrodescorrespond in phase to the electrical signals proudced by soundtranslatlicilig devices disposed in the corresponding acoustic 7. Asound reproduction system comprising a plurality of microphones disposedin a sound field for converting acoustical signals into correspondingelectrical signals, a three dimensional electrically conductive element,means for applying said electrical signals to said element to create anelectrical field therein, said electrical field substantiallycorresponding in phase to said acoustical signals in said sound eld, aplurality of electrodes for electrically contacting a plurality ofpoints on said element, a member for maintaining said electrodes in alxed spaced relationship, a mounting, means for pivotally attaching saidmember within said mounting whereby the positions of said electrodes onsaid three dimensional element may .be varied, a utilization circuit,and means for electrically connecting said plurality of electrodes tosaid utilization circuit.

8. A sound reproduction system comprising a plurality of soundtranslating devices disposed in a sound eld for converting acousticalsignals into corresponding electrical signals, an electricallyconductive element, said element including a container having anelectrically conductive liquid contained therein, means for applyingsaid electrical signals to said container to create an electric eldwithin said electrically conductive liquid, said electric fieldsubstantially corresponding in phase to said acoustical signals in saidsound field, electrodes for electrically contacting a plurality ofpoints within said liquid, means for moving said electrodes to dilerentpoints within said liquid, a utilization circuit, and means forelectrically connecting said electrodes to said utilization circuit.

References Cited in the le of this patent UNITED STATES PATENTS2,305,597 Bauer Dec. 22, 1942

